In mammalian embryos, primordial germ cells (PGCs) are first allocated into extraembryonic tissues and subsequently proliferate while migrating to the fetal gonad. Shortly after colonizing the gonad, germ cells cease dividing and undergo sex specific differentiation. Mice harboring mutations in key growth factors or receptors exhibit PGC defects, demonstrating the necessity of somatic factors for PGC development. Using a cell culture assay which supports several aspects of PGC development, we found that germ cell proliferation can also be inhibited by two related ligands, TGFbeta and activin. The goal of this proposal is to test the hypothesis that these signaling pathways control the proliferation of PGCs in vivo, and thereby contribute to determining the final number of germ cells per embryo. Our first specific aim is to determine whether these signaling pathways individually regulate PGC proliferation. We will measure germ cell proliferation both in vivo and in culture from embryos lacking either activin or TGFbeta receptors. These two receptor systems share a common signal transduction pathway, suggesting that germ cell defects in embryos lacking one receptor system may be functionally compensated by the other system. Therefore, our second specific aim to measure PGC proliferation in embryos lacking both receptors systems in order to fully expose potential defects. Alternatively, these signaling pathways may not be essential for the normal regulation of PGC accumulation, but serve only to inhibit the proliferation of a highly multipotent and migratory cell type in ectopic locations. As a test of this hypothesis, in our third specific aim we propose to use targeted expression of TGFbeta in vivo to determine whether this ligand can prematurely inhibit PGC proliferation. Previous work has shown that PGCs migrate toward a source of TGFbeta in vitro, suggesting that these signaling pathways are part of the mysterious process that guide PGCs to the fetal gonad. Our fourth specific aim is to take advantage of a cell autonomous germ cell marker to determine whether PGCs lacking these receptor systems accumulate in ectopic locations.